Heavy metal ions, such as zinc, are resistant to acid attack. Zinc ranks above hydrogen in the electrochemical series, so that metallic zinc in an acidic solution will react to liberate hydrogen gas as the zinc passes into solution to form di-cations, Zn2+. Zinc has been shown to have antibacterial properties in plaque and caries studies.
Soluble zinc salts, such as zinc citrate, have been used in dentifrice compositions, see, e.g., U.S. Pat. No. 6,121,315, but have several disadvantages. Zinc ions in solution impart an unpleasant, astringent mouthfeel, so formulations that provide effective levels of zinc, and also have acceptable organoleptic properties, have been difficult to achieve. Finally, the zinc ions will react with anionic surfactants such as sodium lauryl sulfate, thus interfering with foaming and cleaning.
Tetrabasic zinc chloride (TBZC), is a zinc hydroxy compound with chemical formula Zn5(OH)8Cl2.H2O. It is also referred to as zinc chloride hydroxide monohydrate, basic zinc chloride, zinc hydroxychloride, or zinc oxychloride. It occurs naturally as the mineral simonkolleite. Unlike zinc chloride, TBZC is insoluble in water. TBZC has been suggested for use in oral care compositions, see e.g., GB2243775A, but such formulations do not deliver zinc efficiently to the teeth due to the insolubility of TBZC.
While the prior art discloses the use of various oral compositions for the treatment of dentinal hypersensitivity, dental caries, and enamel erosion and demineralization, there is still a need for additional compositions and methods which provide improved performance in such treatments.